1. Technical Field
The present disclosure relates in general to drilling offshore wells using dual- and/or multi-gradient mud systems. More particularly, the present disclosure relates to systems and methods for drilling offshore wells using such mud systems, and circulating out influxes, such as, but not limited to influxes known as a “kicks.”
2. Background Art
In conventional (non-dual-gradient) drilling of offshore wells, pore pressure is controlled by a column of mud extending from the bottom of the well to the rig. In so-called “dual gradient” drilling methods, which have been developed over the last ten years to drill in deep and ultra-deep waters, the mud column extends only from the bottom of the hole to the mudline, and a column of seawater or other less dense fluid that exerts a lower hydrostatic head then extends from the mudline to the rig. Kennedy, J., “First Dual Gradient Drilling System Set For Field Test,” Drilling Contractor, 57(3), pp. 20, 22-23 (May-June 2001). These systems use a pump and choke, in some systems a subsea pump and subsea choke manifold or pod, to implement the dual gradient system. The subsea pump is employed near the seabed and is used to pump out the returning mud and cuttings from the seabed and above the BOPs and the surface using a return mud line that is separate from the drilling riser.
Thus there are two broad categories of dual gradient drilling systems: those that use a surface pump and either a surface choke or a subsurface choke (or both) to implement the dual gradient, and those that use a subsea pump and subsea choke manifold (sometimes referred to as a “sensor and valve package”).
In all dual gradient systems, a problem that needs to be addressed is how to remove (or “circulate out”, or simply “circulate”) an “influx” of fluid (gas and/or liquid), such as a “kick”, that has entered the dual gradient drilling fluid.
The methods and systems proposed herein are applicable to the second type of dual gradient drilling methods noted above, i.e., dual gradient methods and systems that use a subsea pump to implement the dual gradient system. Although previous research projects have developed equipment and methodologies to drill wells with dual gradient mud systems, the known systems and methods to drill well bores using dual gradient systems and circulate out any well bore influx in a dual gradient environment have not been satisfactory.
U.S. Pat. No. 6,484,816 (Koederitz) appears to describe a conventional single mud weight situation using surface mud pumps, and not a dual gradient situation employing a subsea pumping system. The reference describes methods and systems for maintaining fluid pressure control of a well bore 30 drilled through a subterranean formation using a drilling rig 25 and a drill string 50, whereby a kick may be circulated out of the well bore and/or a kill fluid may be circulated into the well bore, at a kill rate that may be varied. A programmable controller 100 may be included to control execution of a circulation/kill procedure whereby a mud pump 90 and/or a well bore choke 70 may be regulated by the controller. One or more sensors may be interconnected with the controller to sense well bore pressure conditions and/or pumping conditions. Statistical process control techniques may also be employed to enhance process control by the controller. The controller 100 may further execute routine determinations of circulating kill pressures at selected kill rates. The controller may control components utilized in the circulation/kill procedure so as to maintain a substantially constant bottomhole pressure on the formation while executing the circulation/kill procedure. While this reference does describe shutting in the well bore and circulating a kick out of the well bore using a constant bottom hole pressure using a mud pump 90, and a choke 70 or choke manifold, the description clearly calls for using mud pumps “located near the drilling rig 25” (col. 5, lines 45-50), and not subsea pumps.
U.S. Pat. No. 6,755,261 (Koederitz) has essentially the same description as the '816 patent except that the surface mud pump 90 is controlled to provide a varied fluid pressure in a circulation system while circulating a kick out of the well bore when using a conventional drilling mud. There is no mention of drilling using a dual gradient system, or subsea pumping systems to implement either the dual gradient system, or to circulate out an influx such as a kick.
U.S. Pat. No. 7,090,036 (deBoer) describes a system for controlling drilling mud density at a location either at the seabed (or just above the seabed) or alternatively below the seabed of wells in offshore and land-based drilling applications is disclosed. The system combines a base fluid of lesser/greater density than the drilling fluid required at the drill bit to drill the well to produce a combination return mud in the riser. By combining the appropriate quantities of drilling mud with a light base fluid, a riser mud density at or near the density of seawater may be achieved to facilitate transporting the return mud to the surface. Alternatively, by injecting the appropriate quantities of heavy base fluid into a light return mud, the column of return mud may be sufficiently weighted to protect the wellhead. At the surface, the combination return mud is passed through a treatment system to cleanse the mud of drill cuttings and to separate the drilling fluid from the base fluid. The system described uses a separate “riser charging line 100” running from the surface to a subsea switch valve 101 to inject a base fluid into the returning mud either above the mudline or below the mudline. Importantly, it is noted in the description that “the return mud pumps are used to carry the drilling mud to a separation skid which is preferably located on the deck of the drilling rig. The separation skid includes: (1) return mud pumps, (2) a centrifuge device to strip the base fluid having density Mb from the return mud to achieve a drilling fluid with density Mi, (3) a base fluid collection tank for gathering the lighter base fluid stripped from the drilling mud, and (4) a drilling fluid collection tank to gather the heavier drilling mud . . . .” There is thus no mention of a subsea pumping system to implement the dual gradient drilling method, or circulating a lighter fluid down the drill pipe and into the annulus, keeping a constant bottom hole pressure, while using the subsea choke manifold to control the flow to the subsea pump (and thus the bottom hole pressure).
U.S. Pat. No. 7,093,662 (deBoer) is similar in disclosure to the '036 patent, however, there is no discernable difference between the two descriptions. The '662 patent includes system claims (as opposed to method claims in the '036 patent). As such, the '662 fails to be novelty destroying for the same reasons as the '036 patent.
U.S. Pub. Pat. App. No. 2008/0060846 (Belcher et al.) discloses a method for dual gradient drilling, but does not disclose a subsea pumping system. (In the figures, such as FIG. 2, mud pump 60 is located at the surface.)
U.S. Pub. Pat. App. No. 2008/0105434 (Orbell et al.) discloses an “offshore universal riser system” (OURS) and injection system (OURS-IS) inserted into a riser. A method is detailed to manipulate the density in the riser to provide a wide range of operating pressures and densities enabling the concepts of managed pressure drilling, dual density drilling or dual gradient drilling, and underbalanced drilling. This reference is difficult to understand, but seems to disclose a subsea pumping system in FIG. 3g. Managed pressure drilling is discussed, as is dual gradient drilling, however, there is no discussion of kicks and how to circulate out kicks. The only mention of uncontrolled pressure events (kicks) is in [0048] as follows: “The OURS System allows Nitrified fluid drilling that is still overbalanced to the formation, improved kick detection and control, and the ability to rotate pipe under pressure during well control events.” Therefore, this reference is not enabling to teach methods and systems recited in the present claims, even though a subsea mud pump is disclosed in FIG. 3g. The only discussion of FIG. 3g is as follows, in [0034]: “FIG. 3g shows the system used to enable the DORS (Deep Ocean Riser System)”; and in [0097]: “The OURS and OURS-IS can be used without a SBOP, thus substantially reducing costs and enabling the technology shown in FIG. 3g. This FIG. 3g also illustrates moving the OURS-IS to a higher point in the riser.” There is no disclosure in this reference of diagnosing an influx after shutting in the well to determine if pressure control may be used to circulate the influx out of the well; determining the size of the kick; determining how much the fluid weight will need to be reduced to match the dual gradient hydrostatic head before the influx reaches the subsea pump take point; or circulating a lighter fluid down the drill pipe and into the annulus, keeping a constant bottom hole pressure, and using the subsea choke manifold/“sensor and valve package” to control the flow to the subsea pump (and thus the bottom hole pressure). Nor is there description of pumping sufficient lighter weight fluid into the annulus using a surface pump until the fluid in the annulus has a density less than or equal the density of the balance of the dual gradient system; or isolating the subsea pump and circulating the influx up the drilling riser using the surface pump, through the BOP, and finally out the surface choke manifold.
U.S. Pub. Pat. App. No. 2010/0018715 (Orbell et al.) is a continuation or CIP of the '434 application, and lacks the same features that are lacking in the '434 application.
GB 2 365 044 (Wall et al.) discloses a drilling system which may include a subsea pump to implement a dual gradient drilling method. A light fluid, such as nitrogen, may be injected into a mud return riser. However, the '044 patent does not describe well bore influxes or how to deal with them.
Furlow, W., “Shell Moves Forward With Dual Gradient Deepwater Drilling Solution,” Offshore Int., 60(3), pp. 54, 96 (March 2000), discusses Shell's efforts at dual gradient drilling using a subsea pumping system (SSPS) featuring electrical submersible pumps (ESPs) which were well-known in conventional drilling. The stated goal was to implement dual gradient drilling using as much “established technology” as possible. The use of ESPs was possible because a primary separation of larger drill cuttings and gases from the returning mud upstream of the ESPs was made using subsea separators. Gases are vented subsea. The authors state: “The pumps are not required to handle large-sized materials or high-pressure gas during a well control event.” In discussing the subsea well control, the author states: “The SSPS uses a subsea choke and vents gas at the seabed. As a result, high-pressure containing equipment is only required upstream of the choke. The pump and return conduit systems are not high pressure. When a gas kick is detected, a preventor will close securing the well. As with a conventional system, the driller will receive sufficient information to allow early kick detection, calculation of the proper weight for the kill mud, and the proper drill pipe/volume schedule to adjust the choke and circulate out the kick.” From this description, it is unclear if the author discloses keeping a constant bottom hole pressure, and using the subsea choke manifold to control the flow to the subsea pump (and thus the bottom hole pressure). The authors state that during well control, “the venting pressure is passively controlled to be equal to the ambient seawater pressure”, but this is not the same as maintaining a constant bottom hole pressure.
Kennedy, J., “First Dual Gradient Drilling System Set For Field Test,” Drilling Contractor, 57(3), pp. 20, 22-23 (May-June 2001) describes a joint industry project (JIP) to develop dual gradient drilling employing a subsea mudlift, called subsea mudlift drilling, or SMD. The article describes a test to be conducted on a semi-submersible in a producing field in the Green Canyon area of the Gulf of Mexico. After discussing the difference between conventional drilling and dual gradient drilling, and the advantages of the latter for ultra-deep drilling, the author discusses the components of the SMD, including a drill string valve (DSV), a Subsea Rotating Diverter (SRD) and the Subsea Mudlift Pump. “The Mudlift pumps acts as a check valve, preventing the hydrostatic pressure of the mud in the return lines from being transmitted back to the wellbore. The positive displacement pump unit is powered by seawater, which is pumped from the rig using conventional mud pumps down an auxiliary line attached to the marine riser. The cuttings-laden mud, as well as any other well fluids, will be returned to the rig via another line attached to the riser.” Regarding well control, there are several laudatory, but not too descriptive or enabling remarks: “Drilling efficiency and safety is increased because well kicks and lost circulation problems are reduced and less rig ‘trouble time’ will be experienced” . . . . “Kicks can be circulated out at almost any flow rate”; and “Bottomhole pressure can be varied by adding barite or raising the mud/seawater interface in the riser.” Given the disclosure of this document, while there is mention of dual gradient drilling implemented using subsea pumps, and circulating out kicks is discussed, there is no description of the aspect or feature of maintaining a constant bottomhole pressure while circulating out a kick, or using the subsea choke manifold/“sensor and valve package” to control the flow to the subsea pump (and thus the bottom hole pressure). Nor is there description of pumping sufficient lighter weight fluid into the annulus using a surface pump until the fluid in the annulus has a density less than or equal the density of the balance of the dual gradient system; or isolating the subsea pump and circulating the influx up the drilling riser using the surface pump, through the BOP, and finally out the surface choke manifold.
Regan et al., “First Dual-Gradient-Ready Drilling Riser Is Introduced,” Drilling Contractor, 57(3), pp. 36-37 (May-June 2001) is an article by two of the listed inventors on the above-referenced GB 2 365 044 (Wall et al.), and is largely cumulative of the '044 patent. Indeed, the article actually seems to teach away from the use of subsea pumps (p. 37): “Using a smaller fluid return line increases the velocity of the return flow to 3 times that of the riser without the use of the booster line, making it easier to carry the cuttings out of the well. This would require a high-pressure rotary isolation tool. Combined with nitrogen injection, glass beads or foam, this may eliminate the need for subsea pumps in dual gradient drilling.”
Furlow, W., “Shell's Seafloor Pump, Solids Removal Key To Ultra-Deep, Dual Gradient Drilling,” Offshore Int., 61(6), pp. 54, 106 (June 2001) is a follow-up article to Furlow's 2000 article, and is largely a re-hash of that article. Kick gas is handled by a subsea mud/gas separator. The separator “eliminates free gas before sending returns to the surface, simplifying well control operations and reducing the volume of gas that is handled at the surface near rig personnel.” Accordingly, kicks are not circulated out of the well, but are vented subsea.
Other possibly relevant non-patent literature are Forrest et al., “Subsea Equipment For Deep Water Drilling Using Dual Gradient Mud System,” SPE/IADC Drilling Conference (Amsterdam, Netherlands, Feb. 27, 2001-Mar. 1, 2001) (mentions dual gradient drilling systems and subsea pumping to implement the system) and Carlsen et al., “Performing The Dynamic Shut-In Procedure Because of a Kick Incident When Using Automatic Coordinated Control of Pump Rates and Choke-Valve Opening,” SPE/IADC Managed Pressure Drilling and Underbalanced Operations Conference (Abu Dhabi, UAE, Jan. 28, 2008-Jan. 29, 2008) (discusses the importance of being able to handle kicks during managed pressure drilling and dual gradient drilling using a “dynamic shut-in” procedure, followed by a procedure using an “automatic coordinated control system” to displace the kick, where the automatic coordinated control system operates the main pumps and the choke valve).
From the above, it becomes clear that any effort to combine the teachings of conventional and dual gradient drilling techniques to circulate out influx events would not lead to predictable results, as it is clear that conventional drilling teaches to use constant bottomhole pressure, while dual gradient drilling appears to prefer varying bottomhole pressure when circulating out kicks—teaching away from each other.
Other patent documents discussing dual gradient drilling include U.S. Pat. Nos. 6,328,107; 6,536,540; 6,843,331; and 6,926,101. There are also known so-called “multi-gradient” mud systems, in which beads having density less than a heavy mud are added to a portion of the heavy mud present in a marine riser. Such mud systems are known (using incompressible beads), for example, from U.S. Pat. Nos. 6,530,437 and 6,953,097. Finally, there have been disclosed so-called “variable density” mud systems employing compressible beads, such as described in published U.S. Pat. App. Nos. 20070027036; 20090090559; 20090090558; 20090084604; and 20090091053. Finally, assignee's co-pending application Ser. No. 12/835,473, filed Jul. 13, 2010, discloses methods and systems for running and cementing casing into wells drilled with dual-gradient mud systems include running casing through a subsea wellhead connected to a marine riser, the casing having an auto-fill float collar, and connecting a landing string to the last casing run. The landing string includes a surface-controlled valve (SCV) and a surface-controlled ported circulating sub (PCS). The SCV and PCS are manipulated as needed when running casing, washing it down while preventing u-tubing on connections and prior to cementing to displace mixed density mud from the landing string and replace it with heavy-density mud prior to circulating below the mudline thus maintaining the dual gradient effect. The methods and systems described in the present disclosure are applicable to all of these different types of mud systems, and are generally referred to herein simply as “dual gradient mud systems.”
The patent and non-patent documents referenced in this document are incorporated herein by reference for their disclosure of multi-gradient and variable gradient mud systems, as well as to illustrate prior approaches to the need to circulate out any well bore influx in a dual gradient environment. Although previous research projects have developed equipment and methodologies to drill wells with dual gradient mud systems, the known systems and methods to drill well bores using dual gradient systems and circulate out any well bore influxes in a dual gradient environment have not been satisfactory. It would be advantageous if systems and methods could be developed that allow a subsea choke manifold to control and later isolate the flow of circulating fluid to the subsea pump while circulating out a well bore influx in a dual gradient environment.